This application claims the benefit of Japanese Patent application No. 2000-204179 which is hereby incorporated by reference.
1. Field of the Invention
This invention relates to an objective lens having a long working distance and a magnification of about 50, and more particularly to an objective lens used in optical systems such as microscopes which utilize ultraviolet light.
2. Related Background Art
Objective lenses for an ultraviolet region of around 250 nm wavelength include, e.g., a lens disclosed in Japanese Patent Application Laid-Open No. 7-20385. This lens is an objective lens constituted of i) a first lens group which has cemented lens groups each composed of a convex lens and a concave lens, and ii) a second lens group which comprises two or more cemented lens groups, at least one of which includes a triple cemented lens. Correction of aberrations has been made simultaneously for the visible region and the near-ultraviolet region
An ultraviolet objective lens disclosed in Japanese Patent Application Laid-Open No. 3-188407 also has a high NA (numerical aperture) and a high magnification, where color correction has been made over the range of from the visible region to the ultraviolet region and a part of lenses can be set floating in accordance with service wavelength. Herein, xe2x80x9cfloatingxe2x80x9d is meant to move lens groups so as to make aberrations small in accordance with service wavelength while keeping the length of the whole optical system at a constant value.
In these conventional cases, the lenses can be used under ultraviolet light when used for the inspection of wafers, inspection of pits of, e.g., digital video disks (DVD) and inspections of, e.g., hard disk drives (HDD). However, when used in a microscope incorporated with an automatic focus detector which emits near-infrared light onto an object surface to be observed, to detect focal position on the object surface on the basis of the light which casts back therefrom, it is difficult for the object surface to be focused because the near-infrared light or near-ultraviolet light is not at the same focal position with the ultraviolet light.
The ultraviolet objective lens disclosed in Japanese Patent Application Laid-Open No. 3-188407 also has a disadvantage of poor operability because it is a lens some part of which is set floating in accordance with service wavelength and also it has a very short working distance (WD).
In the case when objective lenses are used in the near-ultraviolet region, some materials constituting an optical system may cause an abrupt fall of transmittance or may cause solarization. Accordingly, there is a limitation on the materials to be used. In particular, where the wavelength is 300 nm or shorter, mediums usable as glass materials are necessarily limited to quartz and fluorite. These optical materials have a small difference in Abbe constants, but the chromatic aberration can sufficiently be corrected by using triplets in a large number as cemented lenses. However, any cement is not available which has a sufficient transmittance at around a wavelength xcex of 250 nm and has a good operability and a superior cementing force. Thus, there has been a problem that those making use of highly precise triple cemented lenses in a large number have had a difficulty in manufacture.
Moreover, the light used for observation and that for automatic detection of focal points are different in wavelength from each other. Hence, because of the above limitations on optical materials to be used, it is difficult to materialize an objective lens in which image planes in respect of the light having wavelength used for observation and the light having wavelength for automatic detection of focal points have been brought into substantial agreement and at the same time a high numerical aperture and a high magnification have been ensured and also the longitudinal chromatic aberration of two wavelengths has been corrected. There has also been such a problem.
The present invention was made taking account of the above problems. Accordingly, an object of the present invention is to provide a microscope objective lens which is durable against its use in the ultraviolet region, near-ultraviolet region and near-infrared region and in which materials having ensured a sufficient transmittance are used and at the same time the longitudinal chromatic aberration and various aberrations have well been corrected, also having a relatively long working distance.
To achieve the above object, the present invention provides a microscope objective lens comprising, in order from the object side, a first lens group, a second lens group and a third lens group;
said first lens group comprising a meniscus lens with its concave surface facing the object side and a plurality of cemented lenses, and having a positive refractive power on the whole;
said second lens group comprising a plurality of cemented lenses and having a positive refractive power on the whole; and
said third lens group comprising a plurality of cemented lenses and having a negative refractive power on the whole;
the lens fulfilling the following conditions (1) to (3):
2.5 less than f1/f less than 5xe2x80x83xe2x80x83(1)
30 less than f2/f less than 70xe2x80x83xe2x80x83(2)
15 less than |f3/f| less than 30xe2x80x83xe2x80x83(3)
where;
f1 is the focal length of the first lens group;
f2 is the focal length of the second lens group:
f3 is the focal length of the third lens group; and
f is the focal length of the whole microscope objective lens system.
According to the present invention, a microscope objective lens can be provided which is durable against its use in the ultraviolet region, near-ultraviolet region and near-infrared region and in which materials having ensured a sufficient transmittance are used and at the same time the longitudinal chromatic aberration and various aberrations have well been corrected, also having a relatively long working distance.